Vacuum plasma processors are used to deposit materials on and etch materials from workpieces that are typically semiconductor, dielectric and metal substrates. A gas is introduced into a vacuum plasma processing chamber where the workpiece is located. The chamber pressure is typically in the range of 0.1 to 1000 torr. The gas is ignited into an RF plasma in response to an RF electric or electromagnetic field. The RF field is provided by a reactive impedance element, usually either an electrode array or a coil which couples both magnetic and electrostatic RF fields to the gas. The reactive impedance element is connected to a first RF source having a first relatively high RF frequency and sufficient power such that the gas is ignited into the plasma. Typically, the gas is introduced into the chamber through the top of the chamber and is withdrawn from the bottom of the chamber. It is common for an electrode at the top of the chamber to be associated with a series of baffles and openings into the excitation region to provide a shower head effect for the gas flowing into the excitation region. The workpiece is usually mounted on an electrode at the bottom of a plasma excitation region in the chamber. In some chambers, the electrode carrying the workpiece is the reactive impedance element supplied with the first RF frequency and a second usually top, electrode spaced from the electrode carrying the workpiece is connected to a reference potential, typically ground. In other chambers the top electrode is supplied with a second RF frequency. It is known to provide such a chamber with an exterior metal wall arrangement at a reference potential, e.g., ground, and a plasma confinement region spaced from the exterior wall, i.e., within the interior of the chamber.
The plasma confinement region includes a structure, such as ring shaped louvers, designed to prevent plasma from flowing from the region and permit uncharged gas molecules to flow from the confinement region. The uncharged gas molecules flow through one or more gaps between opposed facing adjacent surfaces in the vicinity of the periphery of the confinement region. From the gap(s), the uncharged gas flows in a region of the chamber between the confinement region and the chamber wall to an outlet of the chamber connected to a vacuum pump.
A sheath that does not include charge particles and has a thickness determined by the plasma density is formed between the confinement region gap(s) and the plasma. The spacing of the gap(s) between adjacent pairs of the ring shaped louvers is such that the sheath has a thickness that extends entirely through the gap(s) between the adjacent pairs of louvers. As a result, charge particles are not incident on the chamber exterior wall, to (1) provide better control of the plasma in the confinement region and a cleaner chamber outside the confinement region, and (2) reduce damage to the portion of the chamber exterior to the confinement region as result of the plasma not being incident on these portions of the chamber.
It is known, however, that in a typical prior art plasma processor having a confinement region there are occasional losses of plasma confinement, i.e., plasma is present in the portion of the chamber between the exterior of the confinement region and the chamber wall. The loss of plasma confinement is usually due to (1) direct transport of plasma through the gap(s) and/or (2) plasma generation outside the ring shaped louvers, i.e., confinement rings. Reasons (1) and (2) are correlated through the leaking plasma being effectively an RF electrode that carries the RF potential of the plasma through the gap(s) from the confinement region to the chamber region between the exterior of the confinement region and the chamber wall. Presumably, in the absence of plasma leakage through the confinement structure gap(s) there is not enough RF voltage outside the confinement region to ignite the gas outside the confinement region to form a plasma.
To establish a sheath having the necessary thickness to absolutely prevent loss of plasma confinement with the typical prior art confinement structure, the separation between facing surfaces of the gaps between adjacent pairs of louvers is frequently so narrow that there is a considerable gas flow impedance between the interior of the confinement region and the portion of the chamber outside the confinement region. Consequently, the flow rate of gas flowing into and out of the confinement region is frequently less than optimum.
It is, accordingly, an object of the present invention to provide a plasma processor having a new and improved confinement region.
Another object of the invention is to provide a new and improved method of operating a plasma processor to provide enhanced confinement of plasma to a region spaced from the chamber wall.
A further object of the invention is to provide a plasma processor having a confinement region having a lower gas flow impedance, i.e., greater gas flow conductance, between the interior of the confinement region and the portion of the chamber outside the confinement region without adversely affecting plasma confinement within the region.
An additional object of the invention is to provide a new and improved method of operating a plasma processor having a confinement region, wherein the plasma processor is operated so there is a lower gas flow impedance, i.e., greater gas flow conductance, between the interior of the confinement region and the portion of the chamber outside the confinement region without adversely affecting plasma confinement within the region.
An added object of the invention is to provide a plasma processor having a new and improved confinement region with at least one gap, wherein the spacing between opposed facing adjacent surfaces of the gap(s) is increased without adversely affecting plasma confinement within the region.
Yet another object of the invention is to provide a new and improved method of operating a plasma processor having a confinement region with at least one gap, wherein the plasma processor is operated in such a manner that the spacing between opposed facing adjacent surfaces of the gap(s) can be increased without adversely affecting plasma confinement within the region.